A highly sensitive method of detecting infection of live pigs with porcine reproductive and respiratory syndrome virus (PRRSV) was developed by testing alveolar macrophages collected by pulmonary lavage. Five pigs were exposed by oronasal inoculation or by contact to PRRSV when they were 10 (1 pig) or 14 weeks (4 pigs) of age. Diagnostic samples (alveolar macrophages and sera) were collected from each pig just before exposure to PRRSV. During the next 9 weeks sera were collected at weekly intervals and alveolar macrophages were collected at weeks 2 and 4-9. Both sera and alveolar macrophages were suitable for detecting early infection, but alveolar macrophages were clearly the better sample after longer intervals. Virus was last isolated from serum at week 4 (from 1 of 5 pigs), whereas it was isolated from the alveolar macrophages of 4 of the 5 pigs at week 4 and from at least 2 pigs at each of the weekly intervals thereafter (i.e. weeks 5, 6, 7, 8, and 9 postexposure). The most sensitive method of testing alveolar macrophages for PRRSV was cocultivation with MARC-145 cells. None of the pigs had any clinical signs after exposure to PRRSV or as a result of pulmonary lavage and there was no evidence that repeated pulmonary lavage caused anything other than a mild, transient (mild hyperemia) tissue reaction.